1. Field of the Invention
The present invention relates generally to the dislocation free epitaxial growth of materials having significantly large differences in lattice constant and to semiconductor optoelectronic devices such as LEDs, edge-emitting lasers, VCSELs, detectors, and modulators and more particularly to a semiconductor laser comprising a strained material which is grown on a restricted area surface and has a large lattice mismatch between a substrate and an active region in the semiconductor laser.
2. Description of the Prior Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Edge Emitting Lasers (EELs), Light Emitting Diodes (LEDs), photodetectors, or electro-optic modulators (EOMs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, data communications and telecommunications. Vertically emitting or receiving devices have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically emitting devices. The circular nature of the light output beams from VCSELs and surface emitting LEDs also make them ideally suited for coupling into optical fibers as in optical interconnects, data communications, telecommunications or other optical systems for integrated circuits and other applications.
For high-speed optical fiber communications, laser or LED emission wavelengths in the 1.3 .mu.m through 1.55 .mu.m region are desired. Standard silica fiber has zero dispersion near 1.3 .mu.m and has a minimum loss near 1.55 .mu.m. The need for semiconductor lasers emitting in this wavelength region has spawned worldwide development of such lasers. Group III-V semiconductors which emit light in the 1.3 through 1.55 .mu.m region have lattice constants which are more closely matched to InP than to other binary III-V semiconductor substrates such as GaAs. Thus, essentially all commercial emitting lasers emitting at 1.3 through 1.55 .mu.m are grown on InP substrates. These lasers are edge-emitting lasers which, unlike VCSELs, do not require high-reflectivity Distributed Bragg Reflectors (DBRs) to form their optical cavities.
The salient components of a VCSEL typically include two DBRs, and between them, a spacer which contains an active region having a light emitting material. The DBRs and active region form an optical cavity characterized by a cavity resonance at a resonant wavelength corresponding to a resonant photon energy. Unfortunately, it has proven difficult to produce effective DBRs on InP substrates. The available materials which lattice match InP have produced mirrors which are extremely thick and lossy and have thus not resulted in efficient VCSELs.
While epitaxial growth of slightly-lattice-mismatched materials is undertaken routinely, materials which emit in the 1.3 .mu.m through 1.55 .mu.m region have lattice constants sufficiently removed from that of GaAs to make pseudomorphic epitaxial growth problematic. In this context, "pseudomorphic" means having a sufficiently low density of misfit dislocations such that lasers may be produced which have reasonably long lifetimes. For semiconductor lasers, the maximum acceptable density of misfit dislocations or other defects is generally much lower than for other semiconductor devices such as electro-optic modulators or LEDs. The problems have been sufficiently great to cause researchers to abandon such efforts and resort to less desirable hybrid approaches to producing 1.3 .mu.m through 1.55 .mu.m VCSELs.
Thus, the production of VCSELs emitting at 1.3 .mu.m through 1.55 .mu.m wavelengths has been inhibited by either of two problems. The problems result from the fact that VCSELs require laser-quality active materials and high-reflectivity DBR mirrors. These two problems are:
(1) when InP substrates are used, growth of the light emitting active material is straightforward, but production of efficient DBRs is difficult and has not been effective; and PA1 (2) when GaAs substrates are used, DBR production is straightforward, but efforts to grow laser-quality active material have been unsuccessful.
In order to realize such devices, as in the second case above, the misfit dislocations which form as a result of lattice mismatch at high In concentrations or from large epilayer thicknesses must be eliminated. Growth of the strained material on restricted-area surfaces, such as those provided by etching a substrate to form mesas, may allow growth of strained materials to thicknesses well above the Critical Thickness while still keeping free of misfit dislocations. The following is a summary of the prior approaches which are relevant to the problem of producing pseudomorphic semiconductor materials when the materials have a large lattice mismatch.
A detailed, quantitative account of the effects of growth on two-dimensionally-patterned substrates (mesas) is provided by Luryi et al., in an article entitled "New Approach to the High Quality Epitaxial Growth of Lattice-Mismatched Materials," Applied Physics Letters, vol. 49 (July 1986), pp. 140-142. Their analysis assumes an abrupt change of lattice constant. Dramatic increases in critical thickness are predicted, but only for very small pattern widths or very low lattice mismatches. Below a maximum width D.sub.max, the critical thickness is infinite. In other words, the strain force at the interface reaches a saturated level before the onset of misfit dislocations and no longer increases with increasing layer thickness. For a 1% lattice mismatch, D.sub.max is about 2.5 .mu.m (using their FIG. 3 and example of growth of GeSi on Si). Since the mechanical constants of Ge, Si, GaAs, InAs, and InP are not largely different, converting these calculations to InGaAs on GaAs will not cause major departures from the Ge/Si example. At a width of 2D.sub.max, the critical thickness is about twice the nominal critical thickness, i.e., the critical thickness for an infinitely-wide growth surface. Thus, for the example of 1% lattice mismatch, a width of about 5 .mu.m is required for doubling of the critical thickness. For increasing lattice mismatch, the minimum width decreases rapidly. Turning to FIG. 1a, a chart of Lattice Mismatch v. D.sub.max illustrates the area discussed by Luryi et al. and is enumerated by reference numeral 12. As may be seen, the maximum lattice mismatch treated by Luryi et al. is 4% and the largest D.sub.max is at or below 10 .mu.m.
Fitzgerald et al., in an article entitled "Elimination of Interface Defects in Mismatched Epilayers by a Reduction in Growth Area," Applied Physics Letters, vol. 52 (May 1988), pp. 1496-1498, reports growth of In.sub.0.05 Ga.sub.0.95 As layers 3500 .ANG. thick on patterned GaAs in which the number of misfit dislocations was "nearly zero for 25 .mu.m lateral dimensions," i.e., D.sub.max equals 25 um. This article goes on to state that the actual number of dislocations on 25 .mu.m diameter mesas varied between 0 and 3. Therefore, this structure is not always dislocation free. There was an abrupt transition from the GaAs to In.sub.0.05 Ga.sub.0.95 As, and layers were &gt;4 times the CT. Given the very small lattice mismatch of &lt;0.36%, the estimated D.sub.max is about 40 .mu.m by extrapolating the data from Luryi. Turning now to FIG. 1a, this data by Fitzgerald et al. is illustrated by filled circle 14. Fitzgerald et al. also reports that growth on mesas makes it possible to eliminate the generation of misfit dislocations through threading dislocations present in the substrate. The reader is also referred to U.S. Pat. Nos. 5,032,893 and 5,156,995, which discuss the subject matter of the above article by Fitzgerald et al.
Madhukar et al., in an article entitled "Realization of Low Defect Density, Ultrathick, Strained InGaAs/GaAs Multiple Quantum Well Structures Via Growth on Patterned GaAs (100) Substrates," Applied Physics Letters, vol. 57 November 1990), pp. 2007-2009, reported growth of InGaAs/GaAs multiple quantum wells to 2.4 .mu.m total thickness on .about.16.times.18 .mu.m mesas. The average In concentration was about 6.7% (&lt;0.48% average mismatch), and the estimated value of D.sub.max, is 30 .mu.m by extrapolating the data of Luryi. They also report that central regions of the mesas appeared virtually free from structural defects, but the regions within 2-3 .mu.m of the edges did not have good layering. They suspect migration of atoms to be responsible for the poor layering near the edges. Turning now to FIG. 1a, this data by Madhukar et al. is illustrated by filled circle 16.
Since VCSELs are presently the subject of intense research and development, a great deal of results and advancements are published periodically. The following is a list of documents which are relevant to the problem of producing pseudomorphic semiconductor materials when the materials have a large lattice mismatch.
Matthews et al., "Defects in Epitaxial Multilayers I: Misfit Dislocations," Journal of Crystal Growth, vol. 27 (1974), pp. 118-125.
Matthews et al., "Defects in Epitaxial Multilayers II: Dislocation Pile-Ups, Threading Dislocations, Slip Lines and Cracks," Journal of Crystal Growth, vol. 29 (1975), pp. 273-280.
Matthews et al., "Defects in Epitaxial Multilayers III: Preparation of Almost Perfect Multilayers," Journal of Crystal Growth, vol. 32 (1976), pp. 265-273.
Yacobi et al., "Stress Variations and Relief in Patterned GaAs Grown on Mismatched Substrates," Applied Physics Letters, vol. 52 (February 1988), pp. 555-557.
Guha et al., "Defect Reduction in Strained In.sub.x Ga.sub.1-x As Via Growth on GaAs (100) Substrates Patterned to Submicron Dimensions," Applied Physics Letters, vol. 56 (June 1990), pp. 2304-2306.
Chand et al., "Elimination of Dark Line Defects in GaAs-on-Si by Post-Growth Patterning and Thermal Annealing," Applied Physics Letters, vol. 58 (January 1991), pp. 74-76.
Koyama et al., "Wavelength Control of Vertical Cavity Surface-Emitting Lasers by Using Nonplanar MOCVD," IEEE Photonics Technology Letters, vol. 7 (January 1995), pp. 10-12.
In addition, the following U.S. patents may be of interest: U.S. Pat. Nos. 5,512,375; 5,448,084; 5,294,808; 5,156,995; 5,091,767; 5,032,893; 5,019,874; and 4,806,996.
The prior art lacks any means to grow laser-quality materials having large mismatches, e.g. .gtoreq.1%, on mesa sizes appropriate for laser fabrication, e.g. .gtoreq.10 .mu.m, to thicknesses desired for many devices, e.g., well above their critical thicknesses for large-area growth. To achieve .gtoreq.1.3 .mu.m emission on GaAs substrates, lattice mismatches of about 2.5% or greater are required. With 2.5% mismatch, mesa widths less than about 1000 .ANG. are required for thick, defect-free growth, as determined from the data by Luryi. Growth of .gtoreq.2.2 .mu.m-emitting material on InP substrates is expected to require InAs or InAsSb which has &gt;3.2% mismatch from InP. The prior art of patterned substrate growth is thus unable to address the goals of this work in any practical way.
The graded-lattice-constant layers taught by the prior art are completely lacking in ability to produce dislocation-free structures which are well above their respective critical thicknesses. Although references may speak of "almost perfect multilayers," or "device quality" material, at best this refers to an as-grown layer wherein the graded region below has very high defect density. If such layers were used in an active layer for a semiconductor laser, the underlying defects would quickly propagate into the active layer and the laser would be very short lived. This is the case for 1.3 .mu.m lasers grown on GaAs of the type described by Omura et al., "Low Threshold Current 1.3 .mu.m GaInAsP Lasers Grown on GaAs Substrates," Electronic Letters, vol. 25, no. 25, pp. 1718-1719, Dec. 7, 1989; or the structures described by Melman et al., "InGaAs/GaAs Strained Quantum Wells with a 1.3 .mu.m Band Edge at Room Temperature," Applied Physics Letters, vol. 55, pp. 1436-1438, Oct. 2, 1989.